The present invention relates to a Dual Carrier Modulator for a Multiband OFDM transceiver of an ultra wide band (UWB) wireless personal access network.
FIG. 1 shows the transmission of data in a wireless system according to the state of the art. Several transceivers belonging to the same wireless local area network (WLAN) use the same data transmission channel by means of time sharing. At any specific time only one transceiver is transmitting. Accordingly the transmissions from each transceiver are burst like. For helping the receiving transceiver to identify a data transmission burst and for extracting the delivered information data the transmitting transceiver sends a predefined preamble signal preceding the data portion of the data transmission burst. The transceiver that receives the data transmission burst comprises a preamble detection unit that identifies the preamble and thus identifies the data transmission burst. The transceiver uses further the preamble for estimating data transmission and channel parameters such as channel response and carrier and timing offsets that are needed for the data information extraction.
Commonly several communication networks share the same data transmission media. Specifically collocated wireless networks utilize the same frequency spectrum.
FIG. 2 shows two collocated wireless networks according to the state of the art.
Wireless local areas networks (WLAN) represent a new form of communications among personal computers or other devices that wish to deliver digital data. A wireless network is one that does not rely on cable as the communications medium. Whether twisted pair, coax, or optical fibers, hard wiring for data communication systems within a building environment is expensive and troublesome to install, maintain and to change. To avoid these disadvantages wireless networks transmit data over the air using signals that cover a broad frequency range from few MHz to a few terahertz. Depending on the frequency involved wireless networks comprise radio wireless networks, microwave wireless networks and infrared wireless networks.
Wireless networks are used mainly for connecting devices within a building or connecting portable or mobile devices to a network. Further applications are keeping mobile devices in contact with a data base and ad hoc networks for example in committee or business meetings.
Wireless local area networks (WLAN) and wireless personal area networks (WPAN) are used to convey information over relatively short ranges. A wireless personal area network (WPAN) is defined in the IEEE 802.15.3 standard.
In many situations and scenarios several wireless local area networks (WLANs) are operated simultaneously with each other in the same local area. A typical situation would be a big office wherein many office cubicles are located belonging to different divisions of the same company, e.g. search division, accounting division, marketing division. The computers of each division are connected in such a situation by means of separate wireless local area networks (WLANs). A wireless local area network (WLAN) comprising several transceivers is referred to as a Piconet.
FIG. 2 shows typical scenario where two wireless local area networks (WLANs) are operated in the same local area.
In the shown example a first transmitting transceiver A2 transmits data to a receiving transceiver A4 of the first wireless local area network WLANA on the data transmission channel of the wireless local area network WLANA. Further a transmitting transceiver B3 of the second wireless local area network WLANB transmits data to a receiving transceiver B1 of the same wireless local network WLANB on the data transmission channel of this wireless local area network. The data exchange between transceivers is performed half duplex, i.e. a transceiver can either send or receive data over a data link to another transceiver of the same wireless local area network. The data are exchanged via data packets.
Each Piconet WLANi has its respective data transmission channel, i.e. the data transmission channel is used by all transceivers of the corresponding Piconet WLANi.
In most cases the frequency resources available for a wireless local area network WLAN are bounded by regulations. Usually a certain frequency band is allocated for the wireless local networks. Within this frequency band each transceiver is required to radiate no more than a specified average power spectral density (PSD).
To operate several wireless local area networks simultaneously several proposals have been made.
In frequency division multiplexing (FDM) systems according to the state of the art the allocated frequency band is divided into several sub-frequency bands. In FDM-system each data transmission channel and consequently each Piconet is using a different frequency sub-band. Thus, data transmission in different Piconets (WLANs) can simultaneously be performed without interference.
The disadvantage of FDM-systems is that the available capacity for each Piconet is reduced compared to the case where any Piconet is allowed to use the entire allocated frequency band.
The channel capacity is given by the following formula:
  cap  =      ∫                  log        ⁡                  (                      1            +                                          PSD                ⁡                                  (                  f                  )                                                            N                ⁡                                  (                  f                  )                                                              )                    ⁢      df      
The capacity of each Piconet is larger if it will be allowed to use the full frequency band instead of just the allocated frequency sub-band. The reduction in the capacity in FDM-systems translates directly to throughput reduction. Consequently the achievable data bit rate for any specific transmitter-receiver distance is reduced in FDM-systems.
In a CDMA-DSSS (Code Division Multiple Access-Direct Sequence Spread Spectrum) system according to the state of the art a direct sequence spread spectrum is used as a modulation scheme. In DSSS a sequence of many short data symbols is transmitted for each information symbol. In order to support several data transmission channels or Piconets different data sequences with low cross correlation between them are used for different data transmission channels.
In a CDMA-DSSS-system each channel can use the entire frequency band until the maximum possible throughput can be achieved. If some Piconets are working in the same area then the transmission of one Piconet is seen as additional noise by the other Piconets.
The disadvantage of the CDMA-DSSS-System is that there exists a so called near-far problem. When a transceiver in one Piconet is transmitting this transmission will be seen as additional noise by other Piconets. The level of the additional noise is proportional to the cross correlation between the spreading sequences and the received power level of the interferer's signal. For example if the interfering transceiver of Piconet A is close to a receiving transceiver of Piconet B, i.e. closer than a transmitting receiver of Piconet B then the added noise level that the receiving transceiver of Piconet B sees causes a significant reduction in the achievable bit rate for the receiver, so that even a complete blocking of the data transmission channel can occur.
A further proposal according to the state of the art to operate several wireless local area networks (WLANs) simultaneously is to use a CDMA-FH(Code Division Multiple Access-Frequency Hopping)-System. In this CDMA-FH-System the original frequency band is divided into several sub-frequency bands. Any transmitting transceiver uses a certain frequency sub-band for a certain time interval and moves then to the next frequency band. A predefined frequency hopping sequence controls the order of sub-frequency bands such that both the transmitting and receiving transceiver has the information when to switch to the next frequency and to what sub-frequency band.
In a conventional CDMA-FH-System the different data transmission channels are assigned with different frequency hopping sequences.
FIG. 3A shows a CDMA-FH-System according to the state of the art with data transmission channels. A CDMA-FH-System with four data transmission channels can operate four Piconets or wireless local area networks (WLANs) simultaneously at the same local area. In the shown example any transceiver uses a certain frequency band for a transmission interval for 242 ns, remains idle for a predetermined guard time of 70 ns and uses the next frequency band within the next transmission interval etc.
The frequency hopping sequence is fixed for any data transmission channel A, B, C, D. In the given example data transmission channel A has the frequency hopping sequence abc, channel B has the frequency hopping sequence acb, channel C has the frequency hopping sequence aabbcc and channel D has the frequency hopping sequence aaccbb.
A collision is a situation when two transceivers use the same frequency band at the same time. For example a collision between data transmission channel A and data transmission channel B occurs during the first transmission interval when both channels A, B use frequency fa and during the fourth transmission interval when both channels A, B use again frequency fa. A further collision is for example between channel B and channel D during the first transmission interval when both channels B, D use frequency a and the sixth transmission interval when both channels B, D use frequency fb.
When the frequency hopping order of two wireless networks differs two transceivers that belong to different wireless local area networks can transmit at the same time. It may happen that both transceivers use the same carrier frequency at the same time.
One possible CDMA-FH solution is based on OFDM and is called Multiband OFDM. In this case the transceiver transmits a single OFDM in one band and then hops to the next band for transmitting the next OFDM symbol. FIG. 3A depicts 6 OFDM symbols for each channel.
As shown in FIG. 3A the Multiband OFDM Transceiver performs in a time frequency interleaving (TFI) mode band-hopping wherein in each frequency band an OFDM symbol is transmitted. The band-hopping sequence is defined by a TFC code (Time frequency code) stored in a memory. Different collocated networks use different TFC codes. This enables simultaneous transmission of different networks. OFDM symbols from collocated networks collide. In common scenarios the collision level enables efficient communication. Yet in some cases the collisions situation is severe and the communication is not efficient. To overcome severe collisions between transmissions of different networks frequency domain separation (known as FDM) between the wireless networks can be implemented. This is achieved by adding TFC codes with constant band usage (fixed frequency bands). Accordingly a Multiband OFDM Transceiver according to the state of the art is switchable between a time frequency interleaving mode (TFI mode) and a fixed frequency interleaving mode (FFI mode). FIG. 3B shows 7 channels (7 TFC) where 4 channels are of TFI type and 3 channels are of FFI type.
As can be seen in FIG. 3B the transceiver occupies in the TFI mode three frequency bands, wherein each frequency band has a predetermined frequency bandwidth.
According to the evolving multiband OFDM standard the period of one OFDM symbol is 312.5 nSec, i.e. a data length of 242.5 nSec (128 samples at 528 Msps) and a silence time of 70 nSec (37 samples at 528 Msps) between two transmissions.
Consequently the OFDM symbol rate RS=3.2 MHz=1/312.5 nSec. When using three frequency bands there are seven possible time frequency codes (TFC). The first four TFC codes define the frequency band hopping sequence when the transceiver is in the TFI mode. When the transceiver is switched to the FFI mode the transceiver transmits the signal in a fixed frequency band. As shown in the following table and in FIG. 3B, the fifth TFC code indicates that transceiver transmits a signal in a first frequency band, the sixth TFC code indicates that the transceiver transmits the signal in a second frequency band and the seventh TFC code indicates that the transceiver transmits a signal in a third frequency band.
The following TFC codes have three frequency bands as summarized in the following table:
TABLE 1TFC IndexCodeType1[1, 2, 3]TFI2[1, 3, 2]TFI3[1, 1, 2, 2, 3, 3]TFI4[1, 1, 3, 3, 2, 2]TFI5[1]FFI6[2]FFI7[3]FFITFC indices 1–7 in the table corresponds to channels A–G in FIG. 3B
A single burst of transmission is called a PLCP frame. FIG. 4 shows the data format of a PLPC frame used by a multiband OFDM transceiver. Each frame consists of a preamble, a header and a payload data section. The PLPC header is transmitted with a constant data rate of 39.4 Mbit per second whereas the payload data is transmitted with different data rates varying between 53.3 Mbit per seconds and 480 Mbit per second depending on the selected operation mode of the OFDM transceiver. The PLCP frame as shown in FIG. 4 consists of a plurality of OFDM symbols, wherein each OFDM symbol consists of a predetermined number (NCBPS) of encoded data bits. Each OFDM symbol comprises for instance 100 or 200 encoded data bits depending on the selected data rate. As can be seen from FIG. 3B each OFDM symbol is transmitted within different frequency bands fa, fb, fc according to a predetermined frequency hopping pattern. For example three frequency bands fa, fb, fc, are employed by the OFDM transceiver so that seven different frequency hopping patterns are possible as shown in FIG. 3B via a corresponding number of data transmission channels A, B, C, D, E, F, G. Each frequency band fa, fb, fc employed by the OFDM transceiver comprises a center frequency around which a predetermined number of sub-carriers or tones are provided. A frequency comprises for instance 122 sub-carriers consisting of pilot sub-carriers, guard sub-carriers and data sub-carriers. Each sub-carrier is equidistant to its neighboring sub-carrier and can be modulated separately.
FIG. 5 shows an OFDM transceiver according to the state of the art. The transceiver comprises a transmitter and a receiver which are both connected to a higher communication layer block. The OFDM transceiver according to the state of the art as shown in FIG. 5 is a Multiband OFDM transceiver wherein the transmitter transmits OFDM symbols via a data transmission channel and the receiver receives OFDM symbols from said data transmission channel. The conventional transmitter as shown in FIG. 5 is shown in more detail in FIG. 6. The higher communication layer circuit supplies a bit stream to a header generator which adds the header to the payload received from the higher communication layer. The header generator is connected to an error correction encoder which encodes the received data stream. The error correction encoder is connected on its output side to an interleaving circuit. The interleaving circuit interleaves the received bit stream to increase the performance of the data transmission. The interleaved bit stream is applied to a frequency spreading unit within the transmitter. The frequency spreading unit spreads the received bits in the frequency domain with a frequency spreading factor, FSF, which is set according to the data rate applied by the higher communication layer.
The frequency spreading unit is connected to an OFDM symbol modulator which modulates each subcarrier or data tone within the frequency band depending on the data rate or transmission mode. The OFDM symbol modulator according to the state of the art as shown in FIG. 6 comprises a QPSK-Modulator, an IFFT unit performing an inverse fast Fourier transformation and a parallel to serial converter. The output of the OFDM symbol modulator is connected to the time spreading unit of the transmitter which spreads the OFDM symbols in the time domain with a time spreading factor (TSF) depending on the data rate set by the higher communication layer.
Finally the data stream is forwarded to a frequency hopping transmitter which transmits the interleaved and spread OFDM symbols in a different or the same frequency band Fa, Fb, Fc according to a predetermined frequency hopping pattern. The frequency hopping pattern is determined by the selected data transmission channel of the OFDM transceiver.
FIG. 7 shows a conventional receiver within the OFDM transceiver as shown in FIG. 5. The receiver shown in FIG. 7 comprises a frequency hopping receiver to which an OFDM symbol demodulator is connected. The demodulated OFDM symbols are de-spread in the frequency domain and in the time domain by a frequency de-spreading unit and a time de-spreading unit.
The received bit stream is de-interleaved by a de-interleaving circuit and an error correction decoder, e.g. a Viterbi decoder is provided for error correction. Finally the header of the received data packet is extracted by a header extraction unit which recognizes the data rate of the received data stream to adjust the de-interleaving circuit, the time and the frequency de-spreading unit and the error correction unit. The OFDM symbol modulator as provided in the conventional transmitter as shown in FIG. 6 has some serious drawbacks. The OFDM symbol modulator comprises a QPSK-Modulator. The encoded and interleaved binary data is divided into groups of bits and converted into complex numbers representing QPSK constellation points. QPSK-Modulation is used to map groups of 2 coded bits into complex symbols. The OFDM symbols are grouped and sets of pilot tones are added and unused data tones are set to zero. The IFFT unit is provided for converting the signal into the time domain. The output of the IFFT-unit is serialized and transmitted to the air by the frequency hopping transmitter.
Due to the multipath in the wireless media the common response of the data transmission channel is frequency selective. In the transmitter according to the state of the art as shown in FIG. 7 each encoded bit is loaded on one data tone in a single OFDM symbol. If the data tone is corrupted information contained by the encoded bit is changed. When the data tone is completely attenuated by the data transmission channel resulting in a spectral zero or severe tonal signal to noise ratio the information carried by the encoded bit is completely lost. Specifically for high data rates, where no spreading is applied and the code rate is quite high, thus spectral zeros result in performance degradation. Under the selective channel conditions the error correction decoder does not efficiently prevent the loss of bits of severely attenuated data tones. For a given noise level N the signal to noise ratio (SNR) is diminished by the frequency selective channel conditions. The low signal to noise ratio (SNR) of the conventional transmitter according to the state of the art as shown in FIG. 6 causes an increased bit error rate (BER). For a given data rate the OFDM symbol modulator according to the state of the art provided within the conventional transmitter as shown in FIG. 6 does not achieve a reliable communication, i.e. a low bit error rate (BER) when the signal to noise ratio (SNR) of the data transmission channel is low.
Accordingly it is the object of the present invention to provide a modulator for a multiband OFDM transceiver which allows the data transmission over a data transmission channel with a minimum bit error rate (BER) even when the signal to noise ratio (SNR) of the data transmission channel is low.